A source of benzene and xylene is catalytic reformate, which is prepared by contacting a mixture of petroleum naphtha and hydrogen with a strong hydrogenation/dehydrogenation catalyst, such as platinum, on a moderately acidic support, such as a halogen-treated alumina. Usually, a C6 to C8 fraction is separated from the reformate and extracted with a solvent selective for aromatics or aliphatics to produce a mixture of aromatic compounds that is relatively free of aliphatics. This mixture of aromatic compounds usually contains BTX, along with ethyl benzene.
Refineries have also focused on the production of benzene and xylene by transalkylation of C9+ aromatics and toluene over noble metal-containing zeolite catalysts. During the transalkylation of C9+ aromatics and toluene to high value petrochemical products, such as benzene and xylene, over catalysts containing noble metals, by-product saturated compounds are typically produced during the first several months on stream. These by-product saturated compounds can boil in the same temperature range as the desired aromatic products, making separation of the desired products at high purity levels difficult. For example, a benzene product for commercial sale must exceed 99.85% purity. However, initial benzene purity after distillation of a transalkylation reaction product is typically only 99.2% to 99.5% due to the presence of coboilers, such as methylcyclopentane, cyclohexane, 2,3-dimethylpentane, dimethylcyclopentane and 3-methylhexane. Therefore, an additional extraction step is usually required to further improve benzene product purity to the desired level.
One solution to the problem of the production of benzene co-boilers during the transalkylation of heavy aromatics is disclosed in U.S. Pat. No. 5,942,651 and involves the steps of contacting a feed comprising C9+ aromatic hydrocarbons and toluene under transalkylation reaction conditions with a first catalyst composition comprising a zeolite having a constraint index ranging from 0.5 to 3, such as ZSM-12, and a hydrogenation component. The effluent resulting from the first contacting step is then contacted with a second catalyst composition which comprises a zeolite having a constraint index ranging from 3 to 12, such as ZSM-5, and which may be in a separate bed or a separate reactor from the first catalyst composition to produce a transalkylation reaction product comprising benzene and xylene. A benzene product having a purity of at least 99.85% may be obtained by distilling the benzene from the transalkylation reaction product, without the need for an additional extraction step. According to the '651 patent, the second catalyst composition comprises up to 20 wt % of the total weight of the first and second catalyst compositions.
U.S. Pat. No. 5,905,051 discloses a process for converting a hydrocarbon stream such as, for example, a C9+ aromatic compound to C6 to C8 aromatic hydrocarbons, such as xylenes, by contacting the stream with a catalyst system comprising a first catalyst composition and a second catalyst composition, wherein said catalyst compositions are present in separate stages and are not physically mixed or blended and wherein said first catalyst composition is a metal-promoted, alumina- or silica-bound zeolite beta, and said second catalyst composition is ZSM-5 having incorporated therein an activity promoter selected from the group consisting of silicon, phosphorus, sulfur, and combinations thereof. According to the '051 patent, the use of the separate catalytic stages improves the conversion of C9+ aromatic compounds and naphthalenes to xylenes and decreases the amount of undesirable ethylbenzene in the product.
Contrary to the teaching in U.S. Pat. No. 5,905,051, it has now been found that a single stage catalyst system comprising at least two different, specific molecular sieves exhibits enhanced activity for the removal of ethyl-group containing aromatic compounds in C9+ aromatic feeds without overall reduction in the conversion of the C9+ feed to useful compounds, such as xylenes.